1. Field of the Invention
The present invention relates to a variable venturi-type carburetor for use in an internal-combustion engine.
2. Description of the Prior Art
A so-called downdraft variable venturi-type carburetor is known which comprises a substantially vertically-extending intake passage, a suction piston movable in the intake passage in a substantially horizontal direction, a needle fixed onto the tip face of the suction piston, and a fuel passage extending in the axial direction of the needle so that the needle is able to enter the fuel passage. The suction piston in this type of downdraft variable venturi carburetor normally has a flat tip face, and the root portion of the needle is fixed onto the central portion of the flat tip face. However, part of the fuel flowing within the fuel passage flows on the needle, reaches the tip face of the suction piston, and flows downward on the tip face of the suction piston. Consequently, at this time, if the tip face of the suction piston is so formed that it is flat, the flow path of the fuel flowing downward on the tip face of the suction piston fluctuates. If the flow path of the fuel fluctuates, the length of time during which the fuel flows downward on the tip face of the suction piston fluctuates, and, as a result, the amount of fuel fed into the cylinder of the engine fluctuates. In addition, fuel droplets sometimes form while the fuel is flowing downward on the tip face of the suction piston and then intermittently drop off. However, as was mentioned above, if the amount of fuel flowing downward on the tip face of the suction piston and fed into the cylinder of the engine fluctuates, or if the fuel droplets intermittently drop off, the air-fuel ratio is caused to fluctuate considerably, particularly when the amount of air fed into the engine cylinder is small, as when the engine is in an idle state. This results in a problem in that it is difficult to atttain stable operation of the engine.